Molecular sieve materials, both natural and synthetic, have been demonstrated in the past to be useful as adsorbents and to have catalytic properties for various types of hydrocarbon conversion reactions. Certain molecular sieves, such as zeolites, silicoaluminophosphates (SAPOs), aluminophosphates (AlPOs), and mesoporous materials, are ordered, porous crystalline materials having a definite crystalline structure as determined by X-ray diffraction (XRD). Within a crystalline molecular sieve material there are cavities which may be interconnected by channels or pores. These cavities and pores are uniform in size within a specific molecular sieve material. Because the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of industrial processes.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as three-dimensional frameworks of SiO4 tetrahedra and oxides of Group 13 elements (e.g., AlO2) of the Periodic Table. The tetrahedra are cross-linked by the sharing of oxygen atoms with the electrovalence of the tetrahedra containing the Group 13 element (e.g., aluminum) being balanced by the inclusion in the crystal of a cation, for example a proton, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group 13 element (e.g., aluminum) to the number of various cations, such as H+, Ca2+/2, Sr2+/2, Na+, K+, or Li+, is equal to unity.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the “Atlas of Zeolite Framework Types”, eds. Ch. Baerlocher, L. B. McCusker, D. H. Olson, Elsevier, Sixth Revised Edition, 2007, which is hereby incorporated by reference.
The crystallization of molecular sieves, such as aluminosilicates, is often aided by seeding. Seeding is a term commonly used to describe the effect of a minor amount of a product phase (present either through intentional addition or via cross-contamination) in assisting the crystallization of larger quantities of the same material. Seeding can be used effectively for the control of phase purity and crystal size, as well as to accelerate synthesis and reduce or obviate the need for an organic structure directing agent.
It is also known that the synthesis of a crystalline aluminosilicate of one framework type can be assisted by seeds of an aluminosilicate of a different framework type. For example, U.S. Pat. No. 7,067,108 discloses that aluminosilicates of the CHA framework type can be synthesized in the presence of seed crystals of aluminosilicates having an AEI, LEV or OFF framework-type.
In addition, International Publication No. WO 00/06493 discloses that a colloidal suspension of seeds of a molecular sieve of the LEV framework type, such as Levyne, NU-3, ZK-20, ZSM-45 and SAPO-35, can be used to manufacture phosphorus-containing molecular sieves, particularly SAPOs and AlPOs of the CHA framework type.
According to the present invention, it has now been found that the synthesis of aluminosilicate molecular sieves can be seeded with silicoaluminophosphate (SAPO) and aluminophosphate (AlPO) molecular sieves. In spite of the fact that SAPOs and AlPOs are different compositionally from aluminosilicates and SAPO and AlPO seeds do not end up in the final aluminosilicate crystals (no phosphorus incorporation), in some cases no crystalline products are formed without the SAPO or AlPO seeds. It has also been found that there can be structural specificity correlating the seed structure with that of the product aluminosilicate structure, and a clear effect of SAPO and AlPO seeds on the morphology of the aluminosilicate products. This finding is believed to be unprecedented and has the potential for the synthesis of new molecular sieve structures present as a SAPO or AlPO phase but not as a zeolite phase.